Package wastewater treatment plants are pre-manufactured treatment facilities used to treat wastewater in small communities or on individual properties. Here is a outline of this kind of treatment equipment.
What Is A “Package Plant?”
The expression “package plant” derives from the fact that these types of sewage treatment plants are built and assembled in a factory and then shipped and installed “prepackaged” as a complete unit. These package sewage treatment plants became popular throughout the 1960s and are still widely used today. Usually, they are made of 1/4-inch steel and are divided into sections. The components always contain an aeration tank in which the majority of the treatment happens and a clarifier for settling. Some package treatment plants include a sludge waste compartment and chlorine contact tank.
Package plant tanks would be transported on flatbed trucks, unloaded by cranes, and placed on cast-in-place concrete slabs. Several lengths of rebar will extend up through the slab and then be welded to the tank to prevent floating. Another popular tank material is precast concrete. The prefabricated system can be a single storage tank divided into multiple compartments, or it can be a modular system of a larger system. Typical precast concrete tanks are 6’-10’’ x 12’-10’’ or 7’ x 15’. These tanks are cast at the concrete tank maker’s factory and transported by boom-type trucks for off-loading. Generally, after the water tank is installed, the pipes and equipment are set up at the installation site.
What Is An Extended Aeration Package Treatment Plant?
An extended aeration package treatment plant is a package plant that uses extended aeration processes. Extended aeration is an aerobic treatment process in which there is a long residence time to oxidize sewage. The typical residence time of a conventional aeration process is 8 hours. The residence time for prolonged aeration is 24 hours. The extended aeration process can operate efficiently with a dwell time of 12 to 48 hours. The extended aeration treatment is very thorough and can almost completely oxidize organic matter. If the system design is reasonable, nitrification will also occur in the aeration system. The extended aeration system has a high sludge age and generates a large amount of waste activated sludge that needs to be removed regularly.
How Does Sewage Treatment Take Place in An Extended Aeration System?
Whether an extended aeration package treatment plant works effectively depends heavily on the extended aeration system. Extended aeration is a biological treatment process in which microorganisms decompose sewage. There are five basic components necessary for the process:
1. Water - provided by facilities that produce wastewater
2. Sewage - Facilities are also provided; Sewage is microbial food
3. Oxygen - Oxygen is the energy source for microorganisms to metabolize sewage and convert it into itself and carbon dioxide
4. Microorganisms - Microbes naturally exist in sewage. Through the natural selection process, certain organisms will multiply in aeration tanks.
5. A storage tank or container containing the above four components - the storage tank is the “dwelling” or habitat where the organism is to live and the location where the treatment is performed.
The above 5 components must be balanced for proper treatment of an extended aeration package treatment plant. The design of the extended aeration system is critical to its performance and to achieve the expected results of wastewater treatment. Microorganisms use sewage as food and use oxygen provided by blowers as energy. They metabolize sewage and oxygen and absorb a part of the sewage into their own bodies. They will multiply themselves and provide inert harmless by-products of carbon dioxide and water. The number of microorganisms will be controlled by the amount of food and air available and the time available. Too few or too many of these variables can disrupt the balance of the system and reduce the quality of treatment.
What Are Some Other Major Components in Package Sewage Treatment Plants?
Today, most package sewage treatment plants are supposed to further refine the treatment process by installing a tertiary process after the secondary treatment process. Slow sand filters are considered tertiary treatment. Sand filters provide a safety net to capture any solids that may leave the secondary treatment plant. The suspended solids content in filtered wastewater is very low, typically between 1 mg / l and 12 mg / l. Fixed membrane treatments have some additional benefits. In some cases, organisms attach themselves to the sand and provide some additional oxidation and nitrification. The surface of the sand filter will eventually be covered by a layer of biological solids, and water can no longer pass through the solids. When this is achieved, the filter should be taken offline and drained and air-dried for cleaning. After drying, the biomass forms a light skin that can be tilted or shoveled off, placed in a trash can, and finally taken to an approved landfill. Weeds are sometimes a problem in sand filters, and dormant filters may need to be covered with black plastic to kill weeds and seeds.
What Should A Properly Operating Package Treatment Plant Look Like?
A perfect looking extended aeration package treatment plant owns a rich brown colour in the aeration tank. The clarification tank should have a very clear supernatant, and there should be a visible sludge blanket below it. The supernatant depth may vary from 6 inches to 48 inches. The depth of this supernatant will fluctuate during the course of a day based upon the flow speed. Higher flow rates will cause the sludge blanket to rise, thereby reducing the depth of the supernatant.
No two package sewage treatment plants look the same or operate the same. They come in various sizes and configurations. Most people operate these systems well in most situations. If the size is right, it is not easy to flow in and infiltrate, and most importantly to be properly maintained, the expanded aeration plant will work well. Some systems are constantly overloaded and will never function properly because they simply do not work. With proper attention, some systems can operate normally. All systems experience failures and routine failures. These annoyances and malfunctions require immediate identification and correction by service personnel or operators. All processing systems require frequent inspection and maintenance by trained personnel. Running and evaluating package wastewater treatment plants is both science and art. The time and experience of these package treatment plants is ultimately the best teacher.
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